1. Field of the Invention
The present invention relates to a zinc oxide-based conductor, and more particularly, to a zinc oxide-based conductor in which zinc oxide (ZnO) is co-doped with gallium (Ga) and manganese (Mn) to obtain improved electrical conductivity.
2. Description of Related Art
Recently, as a countermeasure against the shortage of energy resources and environmental pollution, the development of high-efficiency solar cells is underway on a large scale. Solar cells are used in various applications such as a power supply for electrical and electronic products, houses, and buildings and an industrial power generator. Technologies, on the basis of which solar cell devices are designed, are considered to have reached a theoretical limit and thus face obstacles to further development. Accordingly, improvement in the efficiency of solar cells by improving the performance of Transparent Conductive Oxides (TCOs), which form the electrodes of solar cells, is gaining more attention.
Indium oxide (In2O3), which contains a small amount of tin as an impurity, is widely used as a TCO. An indium oxide film that contains a small amount of tin as an impurity, i.e., an In2O3—SnO2 based film is also known as Indium Tin Oxide (ITO) film. This film is widely used since it is easy to obtain a low-resistance film. The ITO film satisfactorily realizes high transparency of 80% or more and excellent electrical conductivity of 10−4 Ωcm, required for high-efficiency solar cells.
Extensive research determined that ITO has many merits as a TCO. However, the supply of ITO is instable since indium oxide is a rare metal, obtained as an impurity in the process of refining zinc oxide, and is very expensive. In addition, when a substrate of polymer or the like is used, a TCO should be formed at a low temperature in order to avoid having a harmful thermal effect on a display device or the substrate. However, a film of ITO formed at a low temperature has very low light transparency. In addition, due to weak thermal resistance, the efficiency of a solar cell sharply drops at temperatures of 400° C. or higher. In addition, in the hydrogen plasma process, the high reducibility of In causes chemical instability. Therefore, the development of high-performance transparent conductors that can replace ITO is urgently required, and is becoming a hot issue in the latest research.
As a part of the development of substitute materials, SnO2 and ZnO are gaining attention as the best candidates for the substitute materials for ITO.
Although SnO2 is being distributed and utilized as a transparent conductive material which can substitute for ITO, it has a low light transparency in a long wavelength range (i.e., 900 nm or longer) and deteriorates in the hydrogen plasma process. In addition, it has limits due to material characteristics such as low electrical conductivity, low transparency of about 70%, and the like.
Zinc oxide (ZnO) is a semiconductor material that has a wide band gap (i.e., about 3.3 eV). It has been already been demonstrated that zinc oxide can be made to exhibit excellent transparency (i.e., 80% or more) by doping with aluminum, gallium, or the like. Since zinc oxide is very inexpensive compared to indium oxide, it is being actively studied. It is easy to adjust the electrical and optical properties of zinc oxide using the dopant, since zinc oxide is easy to dope and has a narrow conductivity band. Zinc oxide can be an ideal material in terms of its applicability to transparent conductors, since it is appropriate for coating large area, and is stable in the hydrogen plasma process. In addition, zinc oxide can also advantageously realize a solar cell having a high light-trapping capability by increasing the scattering of incident light, since it is appropriate for controlling surface roughness by texturing.
As examples of dopant added to zinc oxide, gallium (Ga), aluminum (Al), and the like are generally used. In the case of aluminum, the drawback is that an aluminum atom may be bonded with oxygen, and thus easily lose its function as a carrier.
Requirements for the transparent conductor may include excellent electrical conductivity as well as high transparency and light trapping efficiency. In order to obtain high electrical conductivity from Ga-doped zinc oxide, a great amount of dopant has to be added. However, it is impossible to raise its electrical conductivity above a certain value due to its limited solid solubility.
The information disclosed in this Background of the Invention section is only for the enhancement of understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that would already be known to a person skilled in the art.